Rust: Model String -> str implicit conversion in type inference

[hvitved]

Eventually we will want to generally support the Deref trait in type inference, but for now hard-code knowledge about String -> str conversion.

[Copilot]

Pull Request Overview

Adds hard-coded support for implicit String -> str conversion in CodeQL’s Rust type inference, updates related tests, and defines the StringStruct.

• Extends TypeInference.qll to special-case String deref to str when resolving method calls.
• Introduces StringStruct in Stdlib.qll and updates expected outputs across multiple test suites to include as_str targets.
• Adds a demonstration test in main.rs for String -> str conversion via parse.

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

File	Description
rust/ql/lib/codeql/rust/internal/TypeInference.qll	Adds special-case logic for String → str conversion in calls
rust/ql/lib/codeql/rust/frameworks/stdlib/Stdlib.qll	Defines StringStruct mapping to alloc::string::String
rust/ql/test/library-tests/type-inference/main.rs	Adds example test case showing implicit conversion with parse
rust/ql/test/**/PathResolutionConsistency.expected (many)	Updates expected outputs to include new as_str and conversion targets

Comments suppressed due to low confidence (1)

rust/ql/lib/codeql/rust/internal/TypeInference.qll:845

• [nitpick] Consider adding a brief comment above this condition to explain why the StringStruct → Builtins::Str exclusion is needed, to aid future maintainers.

not (

[geoffw0]

We will also want the same feature in data flow access paths at some point, but I guess the logic isn't shared between them.

[geoffw0]

Is it OK that we also still get the result from result = t0 as well in this situation, i.e. that we get two results (of which only one applies depending on context)?

[hvitved]

Yes, that is deliberate.

[redsun82]

LGTM!